The isolation and characterization of temperature sensitive mutants defective in excision repair will be attempted. Such mutants would be desirable since they would afford an interesting comparison with Xeroderma Pigmentosum cells in studies of the relationship of excision-repair to carcinogenesis. The selection procedure is based on utilizing 254 nm irradiation, incorporation of Bromodeoxyuridine in the presence of Hydroxyurea in the post irradiation period at the non-permissive temperature and subsequent irradiation with "black light". Those cells which have incorporated Bromodeoxyuridine are sensitized to "black light" and are killed. The survivors are then shifted to the permissive temperature and allowed to repair normally. These studies are carried on in conjunction with examination of repair endonuclease activity of cells utilizing supercoiled phage DNA as substrate. The DNA can be reacted with a variety of carcinogenic agents without producing breaks in the molecule. These molecules can then be reacted with cell extracts with repair activity to determine the degree of specificity of this activity. Alkaline and neutral sedimentation velocity gradients are used to assay the conversion of supercoiled molecules to nicked molecules.